1. Field of the Disclosure
The present disclosure relates to the field of tactile probing systems for use in and/or with coordinate-measuring machines that measure coordinates of a workpiece. More particularly, the present disclosure relates to a tactile probing system with an improved ability to tolerate excessive displacements.
2. Background Information
Contact probes are known to suffer, as a general matter, due to bending, or alternatively to torsion, tension, and compression. Such contact probes are known to include a sensor system and a stylus with a contact. The sensor system senses when the contact on the stylus contacts a workpiece. The sensor system may include strain gages mounted to thin-walled portions on one or more surfaces. Movable bodies connected to the contact on the stylus are supported by the thin-walled portions, so that displacement of the contact on the stylus is transmitted through to the strain gages mounted to the thin-walled portions. A signal is output when the strain gages are deformed based on displacement of the movable bodies that occurs when the contact on the stylus contacts the workpiece.
The movable bodies of the sensor may suffer from deformation or even breakage, such as when they are not constrained other than by the thin-walled portions and are subject to excessive displacement resulting from the contact on the stylus contacting a workpiece. That is, the thin-walled portions of the sensor have elasticity limits and may not be able to receive the excessive displacement transmitted via the movable bodies. The problem of excessive displacement being transmitted through relatively-unconstrained movable bodies is made worse when the thin-walled portions that receive the excessive displacement are mounted on different surfaces instead of a single common surface.
A stopper may be provided to constrain displacement of the movable bodies, such that when displacement occurs the thin-walled portions do not exceed the elasticity limit. Alternatively, a kinematic connection may be provided to allow a stylus to separate from the sensor if the stylus is placed under excessive displacement. However, even using singular known mechanisms such as stoppers or kinematic connections, contact probes are still known to suffer from bending, or torsion, tension, and compression.
The description of prior art FIGS. 1A, 1B, 1C and 1D as follows is drawn from the description in Netherlands Patent Application number NL 1010894. A conventional contact probe in NL 1010894 includes a probe 1 with which a measurement object is contacted, a probe housing which forms the connection with the coordinate-measuring machine, the bridge 3 which elastically connects the probe to the probe housing, and 4 the measuring system which measures the displacement of the probe in relation to the probe housing.
Noted components of the conventional contact probe are shown in a top view in FIG. 1A, as a cutout of details of FIG. 1A in FIG. 1B, as a circuit schematic of elements in FIG. 1C, and as a physical layout of elements in FIG. 1D.
The stylus and probe tip 4 is connected to the elastic bridge 3 by an intermediate body 5. In an original design prior to Netherlands Patent Application number NL 1010894, the stylus and probe tip 4 were connected to the intermediate body 5 and a moveable base 1 to form a central platform. The central platform of stylus and probe tip 4, intermediate body 5 and moveable base 1 are connected to three elastic rods 3, which guide the motion of the central platform.
In FIG. 1A, the stylus and probe tip 4 and the intermediate body 5 are relatively rigid so that the deformation of these components is small in relation to the deformation of the elastic bridge 3 so as to maintain measurement sensitivity. The elastic elements of the bridge 3 allow a translation in an out-of-plane direction and in rotations. These rotations result in a pseudo translation at the probe tip 4. The bridge 3 enables translation of the probe tip 4 in all directions in relation to a probe housing (not shown). The bridge 3 in FIGS. 1A, 1B, 1C and 1D consists of three elastic elements in FIG. 1A. The elastic elements of the bridge 3 are located in one plane, which eases manufacturing of the bridge 3 and the strain gauges R1, R2, R3 and R4 on the bridge using etching techniques. Each elongated elastic element 3 defines, in close approximation, solely the movement parallel to the longest side of the beam of the bridge 3.
The orientation and position of the intermediate body 5, and therefore the position of the probe tip 4, are read out by strain gauges R1, R2, R3, R4 on the elastic elements of the bridge 3 in such a way that at least all released degrees of freedom can be measured. A strain gauge R1, R2, R3, R4 is an electric or piezoelectric sensor which has an electrical resistance which is dependent on the mechanical strain in the area of the strain gauge R1, R2, R3, R4.
In the bridge 3, four strain gauges R1, R2, R3 and R4 are fitted to each elastic element of the bridge 3 according to FIG. 1B. The strain gauges R1, R2, R3 and R4 are then connected into a complete Wheatstone bridge according to FIG. 1C. Since the elongated elements of the bridge 3 will in most cases be bent into an S-shape, according to FIG. 1D, the change in resistance of the strain gauges R1 and R4 is opposed to the change in resistance of the strain gauges R2 and R3. It then suffices to have strain gauges R1, R2, R3, R4 on the upper side only, in order to simplify the etching process.
The displacement of the probe tip 4 is measured by means of four strain gauges R1, R2, R3, R4 on each beam of the bridge 3, as shown in FIGS. 1B, 1C, 1D. The change in the resistance of the strain gauges R1, R2, R3, R4 is measured in a Wheatstone bridge, as shown in FIG. 1C. Therefore, the conventional probing system can be implemented where displacements of the probe tip 4 can be measured in three orthogonal directions (3D).
As described, the movable bodies of the probe 1, elastic elements 3 and intermediate body 5 are not constrained, so an issue is created when the elastic elements 3 exceed an elasticity limit in response to the amount of displacement. Additionally, the movable bodies and the stylus are made integral using an adhesive or the like, but this then requires changing an entire sensor in order to change out the stylus.